The Deep Physics Behind the Second Law: Information and Energy As Independent Forms of Bookkeeping

Even after over 150 years of discussion, the interpretation of the second law of thermodynamics continues to be a source of confusion and controversy in physics. This confusion has been accentuated by recent challenges to the second law and by the difficulty in many cases of clarifying which formulation is threatened and how serious the implications of a successful challenge would be. To help bring clarity and consistency to the analysis of these challenges, the aim of this paper is to suggest a simple formulation of the deep physics of the second law, and to point out how such a statement might help us organize the challenges by level of seriousness. We pursue the notion that the second law is ultimately a restriction operating directly on the dynamics of information, so the existence of this law can be traced to the need for a system of “information bookkeeping” that is independent of the bookkeeping for energy. Energy and information are related but independent, so the dynamical restrictions for one cannot be derived from those for the other. From this perspective, we also suggest the possibility that the foundation of the second law may be linked to the finite capacity of nature to store information about its own state.